The recording density of magnetic recording apparatuses is expected to reach 1 Tbits/inch2 in the future as a result of development of magnetic head techniques and perpendicular magnetic recording schemes. However, even with the perpendicular magnetic recording scheme adopted, achieving such a high recording density is not easy because of a thermal fluctuation problem.
A high-frequency field assisted recording scheme has been proposed in order to solve the thermal fluctuation problem. In the high-frequency field assisted recording scheme, a magnetic recording medium is locally subjected to an electric field of a high-frequency which is sufficiently higher than a recording signal frequency and which is close to the resonant frequency of the magnetic recording medium. As a result, the magnetic recording medium with the high-frequency field applied thereto resonates to reduce the coercivity (Hc) of the magnetic recording medium to half of the original value. When the high-frequency field is thus superimposed on the recording field, magnetic recording can be carried out on a magnetic recording medium with a coercivity (Hc) and magnetic anisotropic energy (Ku) which are higher than those in the conventional one.
The use of a spin torque oscillator for generation of a high-frequency electric field has been proposed. The spin torque oscillator comprises two magnetic layers, a spin injection layer and an oscillation layer. When a direct current is conducted through the spin torque oscillator via an electrode, the spin injection layer generates spin torque to subject magnetization in the oscillation layer to ferromagnetic resonance. As a result, the spin torque oscillator generates a high-frequency field. The spin torque oscillator is about several tens of nanometers in size. Thus, the high-frequency electric field generated is localized at a short distance of several tens of nanometers from the spin torque oscillator. Moreover, an in-plane component of the high-frequency electric field allows the perpendicularly magnetized magnetic recording medium to resonate efficiently. This enables a significant reduction in the coercivity of the magnetic recording medium. As a result, magnetic recording is carried out only in portions of the magnetic recording medium in which the recording field provided by a main pole is superimposed on the high-frequency field provided by the spin torque oscillator. This allows the use of a magnetic recording medium that is high in coercivity (Hc) and magnetic anisotropic energy (Ku). Hence, the thermal fluctuation problem, which may occur during high density printing, can be avoided.
To provide a high-frequency field assisted recording head, it is important to design and produce a spin torque oscillator which can oscillate stably at a low current density and which can generate an in-plane high-frequency field allowing the medium magnetization to sufficiently resonate.
That is, when a current of an excessively high density is conducted through the spin torque oscillator, heat generation and migration occur to degrade the characteristics of the spin torque oscillator. Thus, the maximum conductive current density is limited. Hence, it is important to design a spin torque oscillator that can oscillate at as low a current density as possible.
On the other hand, to allow the medium magnetization to sufficiently resonate, the intensity of the in-plane high-frequency field is desirably set to at least a certain level compared to the intensity of an anisotropy field (Hk) in the medium. A possible method for increasing the intensity of the in-plane high-frequency field is to increase one of saturation magnetization in the oscillation layer, the thickness of the oscillation layer, and the rotation angle of the magnetization in the oscillation layer. However, all of these methods serve to increase the current density.